Wizz

Tried the search, didn't really find an answer.

My question is. I'm tring to find a correlation between an aftermarket boost gauge and the factory boost gauge.

Example:
0 on aftermarket = 0 on factory
5-6psi on aftermarket = about halfway up on factory
? on aftermarket = 40 on factory.

FYI: S4 TII

Mindspin311

Convert 40inHg to PSI.

eriksseven

iTrader: (6)

haha, the stock boost gauge is purely for looks... It's completely inaccurate, lol.

The only way to know what you're truly boosting is to get an aftermarket gauge.

Wizz

The factory is in mmHg. I'm also tring to validate how close the stock gauge is.

Wizz

I do have an aftermarket boost gauge. Long story short, I'm tring to find why my boost seems a little low. (About 8psi) Before I could reach 10 and the stock gauge would read just under 40. Now my aftermarket reads 8 and factory is still 40. I'm just typing outloud tring to get some ideas.

eriksseven

iTrader: (6)

hmm... Perhaps a miniature boost leak somewhere?

introVert

1 PSI of pressure = 51.71 mmHg (note mm, not inches of Hg)

NZConvertible

FC boost gauges are calibrated in tens of millimetres of mercury (that's what the x10 mmHg means). This unit is the height mercury would rise in a U-tube if you applied pressure at one end when the other end is open to atmosphere. Atmospheric pressure is 760mmHg or 14.7psi, so 1psi is just under 52mmHg.

The scale on the Series 4 boost gauge goes up to "40" meaning 400 mmHg (7.7 psi). The Series 5 boost gauge goes a little higher to "45" or 450 mmHg (8.7 psi). Both gauges go down "-20" (-200 mmHg, 7.9 inHg), but in normal running the actual manifold pressure is much lower than that. Idle is around -400 mmHg; cruise about -250 mmHg and under high-rpm, closed-throttle goes as low as -500 mmHg. The only time you'll see the stock gauge rise off the "-20" mark is under acceleration or when climbing a hill.

Max boost (S4 6.6 psi, S5 8.3 psi) should see both gauges at almost full-scale, but in stock form most cars can't get that high anymore due to clogged cats and worn turbos, but modified cars will easily go off the top of the scale.

slpin

iTrader: (9)

closed throttle high rpm - i have been about to get almost -700

SureShot

Sounds like you have a small pressure leak in the mechanical gauge's hose.

Wizz

That was one of my thoughts.

So I was thinking:
Leak in aftermarket boost gauge/line
Leak in intake (anywhere from AFM to LIM)
Leaking BOV
Malfunction wastegate
Malfunction twin scroll
Maybe because it has been 90+ degrees outside. Air is not as dense.

Am I forgetting anything?

Wizz

That is about right. My stock gauge goes to 40 and my aftermarket goes to about 8. Damn, I'd really like to get my 2psi back. :-(

Wizz

Can anybody think of anything else to check besides what is listed below? Anybody? Please. :-)

RotaryEvolution

iTrader: (30)

sure:


vacuum line blown off


not that uncommon for the lines to pop off under higher than stock boost. i assume you have checked all of the ones you can see though?

Wizz

Yes I have checked. Still looking for the cause of lowered boost.

HAILERS

It's summer now. Heat.

Funkspectrum

iTrader: (5)

could also be intake manifold gaskets wearing out....have you messed with the manifolds any? especially the upper? I know that with a fried UIM gasket, I was only boosting at about 6psi...after replacement, I was boosting at 11...

HAILERS

The boost guage reads out in units of millimeters of mercury. We like to see our pressure readout in psi (pounds/square inch). The conversion factor is .1934. Crunching thru the figures, we get:
5 = 1.0 psi
10 = 1.9 psi
15 = 2.9 psi
20 = 3.9 psi
25 = 4.8 psi
30 = 5.8 psi
35 = 6.8 psi
40 = 7.7 psi

NZConvertible

Isn't it about time that changed?

AUGieDogie

iTrader: (4)

no us americans will never go fully over to metric. We tried it in the late 70s and it just didnt' work. Its a pitty because some of the metric stuff is great! some not so much but hey take what we can. Example everyone in the US knows what 2 liters of fluid looks like because that is how we sell soda. BUT if i told you i had a 1.3 liter displacement in my engine. 90% of those same people would go, "huh??"

Confusion:
Crossing into canada from US. Buying gas. first you need to figure out how many liters per gallon. (3.8?) then convert the current rate of exchange (20%) and then you can figure out how much you spent. lets see $.89/Lt is that expensive?!?!?!?
(Yes)

anyway it will be SLLLLLLLLLLLLLLLLLLLLLLLLLLOOOOOOOOOOOOOOOOOOOOOOO OOOOWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWWW but we will get it.

HAILERS

Sic 'em Augiedogie. Dadgum furriners.

Wizz

Thanks for the replys.
I have found my 2-3 psi! :-) Damn boost gauge is wacky. Tried my buddy's and I have my 10 psi back. At least Autometer (I know :-/) is going to exchange it.

RETed

**** Autometer boost gauges.


-Ted

NZConvertible

I'll second that...

SpAm@FC

iTrader: (4)

0.89$/liter = expensive????

i'm from austria, i spent per liter 95Octan abaut 1.1€, thats ~1,37$... :/

i hate europe, time to go to the states or australia